Remove duplication in reviewer roulette message

What does this Merge Request do and why?

Specify the Danger rules inherited from gitlab-dangerfiles to exclude simple roulette, which is currently causing duplication in the message posted to merge requests.

• Example of duplication: !2309 (comment 758355306)
• Fix: see below 🙂

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.